1. Field of the invention
The invention concerns the reception of digital signals. To be more precise, the invention concerns the equalization of digital signals, in particular to combat intersymbol interference.
2. Description of the Prior Art
The characteristics (amplitude, sign, phase, etc) of a received digital signal depend on a plurality of information symbols transmitted in succession rather than just one symbol. This dependency is due in particular to the multiple paths taken by the transmitted signal to reach the receiver and also, in the case of cellular radio systems, for example, to the modulation technique employed.
It is difficult to recognize each symbol transmitted independently. To achieve this it is necessary to distinguish continuously the contributions of the various symbols in order to retain only one by the process of equalization. The contributions of the other symbols constitute what is known as intersymbol interference.
The invention applies to the reception of all types of digital signal and in particular to mobile reception. The problems arising from intersymbol interference are even more crucial in mobile application because reception conditions are continually varying (with the position and speed of the mobile). The equalization must therefore adapt to cater for such variations.
A particular application of the invention is to equalize signals incorporating learning sequences whose content is known to the receiver, such as those used in the GSM (Groupe Special Mobile) digital cellular radio system, for example. The GSM has drawn up specifications for the pan-European cellular radiotelephone system embodied in its recommendations. Information is transmitted between base stations and mobile station in packets at carrier frequencies in the order of 1 GHz using a GMSK modulator operating at 271 kbit/s.
Each packet transmitted is made up of a sequence of known symbols called the learning sequence and a sequence of symbols varying according to the data transmitted. The learning sequence is at the center of the packet, between first and second data sequences.
Various adaptive equalization methods are already known. In the GSM context, a recursive equalizer may be used whose coefficients are determined by estimating the impulse response of the channel. This method is somewhat unreliable in the presence of low signal to noise ratios, such as occur in the presence of selective fading, which frequently occurs on the radio channel.
It is also possible to use a Viterbi equalizer which operates in two stages:
it estimates the sequence of data symbols using a maximum likelihood criterion,
it subtracts the intersymbol interference.
The first stage is relatively costly in terms of processing time: this time is proportional to 2.sup.Nr where Nr is the impulse response duration expressed as a number of symbol periods.
A particular objective of the invention is to alleviate these drawbacks of the prior art.
To be more precise, an object of the invention is to provide an equalization method and device to combat intersymbol interference which are efficient and perform well under all receiving conditions, in particular in the presence of selective fading.
Another object of the invention is to provide a method of this kind which is relatively simple to implement and relatively economical in processing time as compared with known methods such as the Viterbi equalizer.
Another object of the invention is to provide a method of this kind which simultaneously estimates the symbols transmitted and eliminates intersymbol interference.
Another object of the invention is to provide a method and a device applicable to mobile reception, and in particular in radio communication systems. Another object of the invention is to provide a method and a device which are entirely compatible with the various GSM recommendations.